Method for recycling argon

ABSTRACT

A method for recycling argon from an industrial process; in which the gaseous argon is compressed after the use thereof in the industrial process, is fed to a main heat exchanger and is cooled there in contact with a first cooling medium. The compressed and cooled argon is fed to a rectification column or another cryogenic separating device and is liquefied there by direct heat exchange with a second cooling medium and is freed of low-boiling substances by rectification. The liquefied argon is drawn from the bottom of the rectification column and, after use as the first cooling medium in the main heat exchanger, is fed back into the industrial process, the raw argon being brought into thermal contact with cryogenically liquefied pure argon in the main heat exchanger and/or the rectification column. The product argon that results is highly pure and can be fed back to the industrial process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national stage application of International Application No. PCT/EP2019/065693, filed Jun. 14, 2019, which International Application was published on Feb. 6, 2020, as International Publication WO 2020/025214 in the German language. The International Application claims priority to German Application No. 10 2018 006 002.1, filed Jul. 28, 2018. The International Application and German Application are hereby incorporated herein by reference, in their entireties.

FIELD

The invention relates to a method for reutilizing argon.

BACKGROUND

There are various methods for purifying gas streams contaminated with foreign substances, and these are employed as a function of the particular case. For example, scrubbing and condensation processes in which the contaminated gas stream is brought into direct or indirect thermal contact with a refrigeration medium, resulting in the contaminating substance condensing out, are used. In adsorption and chemisorption processes, the contaminating substance becomes attached to a medium which is subsequently freed of the contaminating substance by desorption. Methods of this type which serve, in particular, for reducing pollutant emissions or recovering particular constituents are known from, for instance, EP 0 537 473 A1, EP 0 827 770 A2, EP 1 366 793 A1 or EP 1 602 401 A1.

EP 0 331 028 B1 describes a method for purifying crude argon obtained in the cryogenic fractionation of air. Here, a stream of argon which is still contaminated with small proportions of oxygen and nitrogen is subjected, in succession, firstly to treatment with hydrogen gas for reducing the oxygen present in the gas stream and subsequently to rectification for separating off the nitrogen. However, this method cannot readily be used for purifying process gas streams.

The known methods are often very complicated and associated with a high consumption of energy. In some cases, problematical secondary materials, for example wastewater, nitrogen oxides or carbon dioxide, can additionally be formed.

Argon is used in many industrial processes, for example in the field of pyrometallurgy. A freshly produced steel alloy can be degassed and at the same time homogenized by means of argon blowing; in particular, the undesirable, dissolved nitrogen is removed from the melt here. Further applications are, for example, the utilization of argon as inert gas or flushing gas for heat treatment or in particular processes of semiconductor manufacture or in the chemical industry.

The argon used in such processes is generally kept in stock at low temperature in the liquefied state at a pressure of, for example, from 5 bar to 20 bar in a tank and vaporized before use. During its use, the argon still present in very high purity in the tank becomes contaminated with various substances whose boiling points are in some cases lower than the boiling point of argon at the same pressure, for example nitrogen, carbon oxide or hydrogen (such components will hereinafter be referred to as “low boilers”), but sometimes also with higher boiling points, for example oxygen or organic substances (hereinafter referred to as “high boilers”). The argon stream contaminated with these substances can subsequently no longer be used in the same industrial process; in general the argon is therefore subsequently discharged into the atmosphere, which leads to a high consumption of argon. The work-up and recovery of the argon is, however, associated with high costs and consumption of energy and is therefore in most cases uneconomical at present.

SUMMARY

It is therefore an object of the invention to provide a method for reutilizing argon for use in an industrial process, which makes do with a relatively low consumption of energy.

This object is achieved by a method having the features of claim 1. Advantageous embodiments of the invention are indicated in the dependent claims.

The method of the invention for reutilizing argon which is kept in stock in the liquid state in a tank (hereinafter referred to as “pure argon”), taken from the tank, vaporized and fed in gaseous form to an industrial process is characterized in that the gaseous argon which has been used in an industrial process (hereinafter referred to as “crude argon”) is compressed in a compressor, the compressed crude argon is fed to a main heat exchanger and cooled there by indirect heat exchange with at least one first cooling medium, preferably to a temperature above the boiling point of argon at the pressure prevailing in the main heat exchanger, the compressed and cooled crude argon is fed to a separation device and there cooled by preferably direct heat exchange with a second cooling medium to a temperature above the boiling point of argon at the pressure prevailing in the separation device, i.e. at the pressure exerted on the crude argon, resulting in a gas phase composed of argon contaminated with low boilers and a liquid phase composed of argon which has been at least largely freed of low boilers (hereinafter referred to as “product argon”) being formed, the gas phase is at least partly taken off from a head space of the separation device, the product argon is taken off in the liquid state from a bottom region of the separation device, depressurized and used in the main heat exchanger as cooling medium for cooling the crude argon and is vaporized, the vaporized product argon is at least partly recirculated to the industrial process, where pure argon is taken from the tank and brought into direct or indirect thermal contact with the crude argon in order to cool the crude argon in the separation device and/or during feeding of the crude argon to the separation device.

According to the invention, the argon stream coming from the industrial process (here referred to as “crude argon”) is thus freed of low boilers by means of a distillation or rectification process and subsequently, optionally after going through a further purification stage in which high boilers are separated from the argon stream, fed at least partly as purified argon (here referred to as “product argon”) back to the industrial process. The cold required for the purification process is provided entirely or partly by vaporization of pure argon which in any case has to be vaporized for use in the industrial process.

In the main heat exchanger, indirect thermal contact of the compressed crude argon with at least one first cooling medium occurs. As a first cooling medium, use is made of product argon, with further cooling media being optionally able to be additionally used, for example pure argon. This results in cooling of the crude argon to a temperature above or below the boiling point of argon at the pressure prevailing in the main heat exchanger, i.e. at the pressure exerted on the crude argon in the main heat exchanger. In the separation device, direct or indirect thermal contact of the crude argon with a second cooling medium occurs. The second cooling medium is either liquid pure argon which is, for example, fed into the head space of the separation device and brought into direct contact with the crude argon, and/or a medium which has been cooled indirectly by liquid pure argon, for example a substream of the gas phase which accumulates in the head space of the separation device and is discharged from the latter and is at least partly liquefied by indirect heat exchange with cold pure argon and is recirculated to the head space of the separation device.

As a result of the method of the invention, the amount of fresh pure argon which has to be supplied to the industrial plant in which the industrial process is carried out is considerably reduced, leading to not inconsiderable costs being saved. In addition, direct or indirect pollutant emissions associated with the pure argon consumption in the industrial plant, including emissions in the production and transport of the pure argon, are avoided.

Here, a “separation device” is a device in general which allows the above-described cryogenic separation of crude argon into a gas phase contaminated with low boilers and a liquid phase which has been at least largely freed of low boilers. In particular, the separation device is a rectification column or a condenser.

There are various possibilities within the scope of the invention for using the cold content of the pure argon. Those described below are considered to be particularly advantageous:

Firstly, the liquid pure argon taken from the tank can be fed into the head space of the separation device. In this case, direct contact of the pure argon fed in with the crude argon to be purified in the separation device thus occurs. The pure argon thus functions here as “second cooling medium” for cooling the crude argon. In order to achieve a sufficiently low temperature for formation of a liquid phase in the separation device, additional cooling is necessary. For this purpose, the cold content of the product argon, for example, which is depressurized and brought into indirect thermal contact with the crude argon is utilized. Configurations for achieving direct contact of crude argon and pure argon are simple to realize, but are advisable only when no further, relatively high boiling constituents are to be removed from the argon stream or when these have already been removed from the crude argon in a preceding purification stage.

In other advantageous embodiments of the invention, the cold content of the pure argon is utilized by the pure argon entering into indirect heat exchange with the crude argon. During the indirect thermal contact, the pure argon vaporizes without taking up impurities from the crude argon. In addition, it is possible in this embodiment to decouple the pressure conditions of crude argon and pure argon in the heat transfer. In particular, the crude argon can be effectively cooled by the pure argon being present at a lower pressure than the crude argon in the indirect heat exchange. For example, the pure argon is kept in stock in the tank at a pressure which is from the beginning lower than the pressure of the crude argon after compression thereof (and before thermal contact with the pure argon), or the pure argon from the tank is depressurized to an appropriately low pressure before thermal contact with the crude argon.

A preferred embodiment of this type with indirect heat transfer from the crude argon to the pure argon is characterized in that a substream of the argon contaminated with low boilers which is taken off from the head space of the separation device is fed to an overhead condenser in which it is cooled by indirect heat exchange with a third cooling medium and subsequently fed back to the separation device, with pure argon taken from the tank being used as “third cooling medium” in the overhead condenser. The substream which has been cooled by the “third cooling medium” thus serves as “second cooling medium” for cooling the crude argon in the separation device. The overhead condenser is a heat exchanger which is arranged fluidically upstream of the introduction of a medium, here thus the sub stream of the argon contaminated with low boilers, into the head space of the separation device.

In another advantageous embodiment of the invention, indirect heat exchange between pure argon from the tank and the crude argon occurs in the main heat exchanger. The pure argon is thus used here as “first cooling medium” in the main heat exchanger for precooling the crude argon.

Furthermore, another cooling medium or a plurality of other cooling media can be used instead of the pure argon or in addition to the latter for precooling the crude argon in the main heat exchanger or in the overhead condenser and subjected to indirect heat exchange with this crude argon, for example residual gas taken off from the head space of the separation device and/or another cryogenic refrigerant, for example liquid nitrogen or liquid oxygen.

Furthermore, it is of course possible to combine two or more of the abovementioned embodiments for transfer of heat from the crude argon to the pure argon with one another and, for example, use the pure argon as cooling medium both in the overhead condenser and in the main condenser.

The crude argon which has been used as cooling medium in the overhead condenser and/or the main heat exchanger is preferably fed to the same industrial process into which the product argon is also recirculated. However, at least a substream of the pure argon used for this purpose can also be supplied to other industrial processes in which particularly pure argon is demanded.

If the crude argon is not contaminated only with low boilers but also with high boilers, it is necessary for these also to be removed from the argon stream before reutilization of the latter in the industrial process. For this reason, a preferred embodiment of the invention provides for the product argon being fed to a device for separating off relatively high-boiling constituents and being freed of high boilers in this before recirculation to the industrial process. This purification stage, furthermore, can be carried out before or after the purification stage for removing low boilers. To free the crude argon of high boilers, it is possible, according to the invention, to use all known methods for gas purification, for example a cryogenic gas purification process in which the high boilers are condensed out of the argon stream by prefer-ably indirect heat exchange with a cryogenic cooling medium and subsequently discharged. Here too, pure argon from the tank is preferably used as cooling medium, but it is also possible within the scope of the invention to use other liquefied gases as cooling medium, for example liquid nitrogen or liquid oxygen.

After the two purification stages, the product argon which has then been freed of high and low boilers can be fed together with pure argon which has been used as cooling medium in one of the purification stages to the industrial process. In the ideal case, the pure argon used for cooling is sufficient to compensate completely for any argon losses which occur during the course of the industrial process.

The method of the invention is suitable for purifying streams of argon which have a high or low level of contamination with low-boiling substances. For example, the argon stream to be purified has a proportion of argon in the range from 20% by volume to less than 99% by volume, preferably from 40% by volume to 90% by volume, particularly preferably from 50% to volume to 85% by volume, with the respective balance consisting at least predominantly of low-boiling components such as nitrogen and/or hydrogen, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

Working examples of the invention will now be explained in more detail with the aid of the drawings. FIGS. 1-3 schematically show flow diagrams of three working examples of the method of the invention.

DETAILED DESCRIPTION

In the working example 1 shown in FIG. 1, crude argon 2 is obtained in an industrial process 3 in which the crude argon 2 has previously been contaminated with low boilers and high boilers. The industrial process 3 is, for example, a heat treatment of metallic workpieces, in which the crude argon has been used as inert gas or blowing gas. The crude argon 2 is compressed in a compressor 4 to a pressure of, for example, from 5 bar to 20 bar, goes through a main heat exchanger 5 in countercurrent to a number of cooling media, resulting in it being brought to a temperature which is, for example, just above the boiling point of argon at the pressure prevailing on the crude argon side in the main heat exchanger 5. Finally, the crude argon 2 is introduced into the bottom region of a cryogenic separation device, which is a rectification column 6 in the working examples shown here. In the rectification column 6, the crude argon 2 is cooled further by thermal contact with a cooling medium and separated into its constituents, resulting in liquid argon which has been largely freed of low boilers (product argon) collecting in the bottom region 7 of the rectification column 6 while part of the argon and low boilers ascend in gaseous form to the head space 8 of the rectification column 6.

Pure argon 9, which is kept in stock in the liquefied state in a tank 10, is used for cooling the process in the rectification column 6. The pure argon is taken from the tank 10 and fed in working example 1 as liquefied gas into the head space 8 of the rectification column 6.

The low-boiling gas constituents which collect in the head space 8 are discharged together with a relatively small amount of argon as residual gas 12. A substream 13 of the residual gas 12 is fed to an overhead condenser 14 in which it is liquefied and introduced back into the head space 8 of the rectification column 6. The remaining residual gas 12 is used as cooling medium for precooling the crude argon 2 in the main heat exchanger 5 and subsequently released into the surrounding atmosphere or optionally passed to a further use.

The argon which collects in the bottom region 7 of the rectification column 6 is discharged as pure, liquefied product argon 15. It goes through a depressurization valve 16, is cooled in the process and is subsequently used in the overhead condenser 14 for cooling the substream 13. Finally, it goes into the main heat exchanger 5 in which it serves, in addition to the residual gas 12, as cooling medium for precooling the crude argon 2. The now warmed product argon 15 is subsequently fed to a device 18 for removing high boilers from the product argon 15. If no high boilers are present in the crude gas stream 2 or these do not interfere in the subsequent process, passing the product argon 15 through the device 18 becomes superfluous. The device 18 can, furthermore, also be arranged fluidically upstream of the main heat exchanger 5.

The device 18 can encompass various gas purification methods (depending on type and concentration of the impurity), for example a condenser or a plurality of condensers in which the product argon 15 is cooled by thermal contact with a cooling medium to a temperature at which the high boilers present therein condense out. The product argon 15 itself remains in the gaseous state in this operation. The cooling medium used in the device 18 is, for example, likewise liquid pure argon from the tank 10 which is taken off there at a connection port 19 or is another cooling medium, for example liquid nitrogen.

The product argon 15 which has now been substantially freed of low boilers and high boilers is now fed as pure argon back to the industrial process 3 and used there.

In the working examples 20, 22 shown in FIG. 2 and FIG. 3, in which constituents having the same effect are denoted by the same reference numerals as in working example 1, crude argon 2 from an industrial process 3 is also freed of contaminating substances and fed back to the industrial process 3.

In the working example 20 shown in FIG. 2, the pure argon 9 from the tank 10 is, in contrast to working example 1, not introduced into the head space 8 of the rectification column 6 but instead the pure argon 9 travels as cooling medium for indirect heat exchange with the substream 13 through the overhead condenser 14 and at least partly vaporizes there. It is subsequently fed to the main heat exchanger 5 as cooling medium for indirect heat exchange with the crude argon 2. Otherwise, the process engineering structures and operations in working example 20 are essentially the same as in working example 1.

The cold content of the pure argon 9 is thus utilized only for indirect heat transfer in the heat exchangers 5 and 14 in the embodiment 20. The pure argon 9 from the tank 10 does not come into direct contact with the crude argon 2. This makes it possible to stock the pure argon 9 in the tank 10 at a pressure which is lower than the pressure of the crude argon 2 after compression thereof, for example at a pressure which corresponds approximately to the pressure in the industrial process 3. When the pure argon 9 is kept in stock in the tank 10 at a higher pressure, for example at a customary tank pressure of from 5 bar to 20 bar, it is advisable to depressurize the pure argon 9 at a depressurization valve 21 and thus bring it to a lower temperature before it passes through at least one of the heat exchangers 14, 5. For example, as shown in FIG. 2, the pure argon 9 which has been warmed in the heat exchangers 5, 14 is fed to the industrial process 3 or mixed with the product argon 15 downstream of the device 18, or it can be utilized in another way.

In the working example 22 shown in FIG. 3, too, only indirect heat exchange between the pure argon 9 from tank 10 and the crude argon 2 occurs. This occurs exclusively in the main condenser 5 in which the pure argon 9 functions, together with the residual gas 12 and product argon 15, as cooling medium. Here too, the process engineering structures and operations are essentially the same as in working example 1 or working example 20.

LIST OF REFERENCE NUMERALS

-   -   1. Working example     -   2. Crude argon stream     -   3. Industrial process     -   4. Compressor     -   5. Main heat exchanger     -   6. Rectification column     -   7. Bottom region     -   8. Head space     -   9. Pure argon     -   10. Tank     -   11. -     -   12. Residual gas     -   13. Substream     -   14. Overhead condenser     -   15. Product argon     -   16. Depressurization valve     -   17. -     -   18. Device for removing high boilers     -   19. Connection port     -   20. Working example     -   21. Depressurization valve     -   22. Working example. 

1. A method for reutilizing argon in an industrial process, in which argon kept in stock in the liquid state in a tank (pure argon) is taken from the tank, vaporized and fed in gaseous form to an industrial process, wherein: a. the gaseous argon which has been used in the industrial process (crude argon) is compressed in a compressor; b. the compressed crude argon is fed to a main heat exchanger and cooled there by indirect heat exchange with at least one first cooling medium; c. the compressed and cooled crude argon is fed to a separation device and cooled there by heat exchange with a second cooling medium to a temperature below the boiling point of argon at the pressure prevailing in the separation device, resulting in a gas phase composed of argon contaminated with low boilers and a liquid phase composed of argon which has been at least largely freed of low boilers (product argon) being formed; d. the gas phase is at least partly taken off from a head space of the separation device; e. the product argon is taken off in the liquid state from a bottom region of the separation device, depressurized and used in the main heat exchanger as cooling medium for cooling the crude argon, resulting in it vaporizing; f. the vaporized product argon is at least partly recirculated to the industrial process; g. where liquid pure argon is taken from the tank and brought into direct or indirect thermal contact with the crude argon in order to cool the crude argon in the separation device and/or during feeding of the crude argon to the separation device.
 2. The method as claimed in claim 1, wherein the separation device is a rectification column or a condenser.
 3. The method as claimed in claim 1, wherein the pure argon taken from the tank is fed into the head space of the separation device.
 4. The method as claimed in claim 1, wherein argon enriched with low boilers is at least partly taken off from the head space of the separation device, fed to an overhead condenser, cooled there by indirect heat exchange with a third cooling medium and subsequently fed back into the separation device, with pure argon taken off from the tank and/or depressurized product argon being used as third cooling medium in the overhead condenser.
 5. The method as claimed in claim 1, wherein pure argon taken off from the tank is used as cooling medium in the main heat exchanger.
 6. The method as claimed in claim 3, wherein the pure argon which has been used as cooling medium in the overhead condenser and/or the main heat exchanger is fed to the industrial process.
 7. The method as claimed in claim 1, wherein the product argon is fed to a device for separating off relatively high-boiling constituents before it is recirculated to the industrial process.
 8. The method as claimed in claim 4, wherein the pure argon which has been used as cooling medium in the overhead condenser and/or the main heat exchanger is fed to the industrial process. 